Mass spectrometers may be used to measure isotope ratios, for example in carbon or oxygen containing compounds. This may be done, for example, to ascertain a geochemical origin of a sample. The compounds themselves, or their fragments, or their reaction products (e.g. their combustion or oxidation products) are ionised and the masses and the abundances of the resulting ions are measured. In this way, the ratio of isotopes present in the compounds such as 13C/12C or 18O/16O may be determined. This may be performed using solid, gaseous or liquid samples containing the compounds to be analysed, which are typically subject to a separation process prior to ionisation, such as by gas chromatography or liquid chromatography.
Accurate and precise isotope measurements are usually determined on magnetic sector mass spectrometers. However, recently it has been shown that Orbitrap™ mass spectrometers are capable of measuring precise and accurate isotope ratios (John Eiler, presentation at Clumped Isotope Workshop, January 2016; John Eiler et al. Poster at ASMS 2016 conference).
The guiding, and where present, confinement of ions produced in this process leads to space charge effects that have been found to affect the measured isotope ratios. That is, space charge effects affect the fractionation of the ions from the sample leading to different relative abundances of the isotopes being introduced into the mass spectrometer from their true relative abundances present in the compound. This leads to different isotope ratios being measured from one sample to the next due to variations in the experimental conditions.
A known technique for measuring isotope ratios is known as sample/reference bracketing and involves mass spectrometer measurements of ions generated from both a sample to be analysed and also from a reference with a known isotope ratio in an alternating manner (i.e. sample measurements are bracketed by reference measurements before and after). Space charge effects can lead to erroneous isotope ratio determinations, even for the known reference. As the space charge effects can vary from one fill of an ion storage device of the mass spectrometer with ions to the next fill, even the sample/reference bracketing technique cannot be used to correct these effects on the isotope ratio measured for the sample to be analysed. The present inventive concepts seek to address the problems of inaccuracies introduced into isotope ratio measurements because of space charge effects.